It is frequently desirable for lasers to produce a series of optical pulses of high peak power rather than emit the radiation continuously or in free-running (i.e. long pulse) mode. Such a series of pulses might be desirable for example, for surgical applications, dental applications, for welding, or for remote sensing applications. It is also desirable if the pulses can be obtained on demand, such as responsive to some externally applied electrical trigger signal to the laser's control circuitry.
In order for the laser to produce a series of Q-switched pulses in a controlled manner rather than operate in continuous wave (CW) or free-running mode, an optical modulator is usually placed in the laser cavity in order to modulate the optical loss (or 1/Q) of the cavity. The optical modulator is designed to have low loss in the “ON” state, and high loss in the “OFF” state, and is generally referred to as a Q-switch. The laser gain medium is pumped with the Q-switch in the “OFF” state allowing population inversion to reach a high level. In response to a suitable external trigger signal to the Q-switch driver electronics, the Q-switch assumes the “ON” state, allowing the Q of the laser cavity to rise to a high value. The result of the high Q state is a high intensity optical pulse exiting from the output coupler (mirror) of the laser cavity.
Q-switches for lasers operating in the infrared spectral region are usually one of three main types: (1) acousto-optic, (2) electro-optic or (3) mechanical. The mechanical type comprises a spinning mirror or prism, vane or shutter. Mechanical Q-switches are slow in the sense that pulses generated via this approach have long rise and fall times compared to other types of Q-switches. Furthermore, the moving parts often cause vibration and/or reliability problems. Long rise and fall times in a Q-switch are often undesirable as they limit the ability of the laser system to produce short pulses (<10 ns) with sufficiently high peak power for many applications.
Electro-optic Q-switches can be very fast, and generally provide good performance. However there are a limited number of suitable single-crystal electro-optic materials, which can operate efficiently in certain spectral bands, such as the 2-8 micron region. In addition, suitable available electro-optic materials (such as CdTe) are expensive and not easily obtainable in the form of large homogeneous single crystals, as required for the construction of these devices. The optical damage thresholds and thermal conductivities of these otherwise suitable materials tend to be low, thus making the design and fabrication of reliable Q-switches a difficult if not an impossible task.
Regarding acousto-optic modulators, the basic structure of acousto-optic modulators and the form of the acousto-optic interaction is well known. With reference to FIG. 1(a), a acousto-optic modulator 100 is shown which includes an acousto-optic material 110 having a transducer (T) 105 bonded thereto. The transducer electrodes are not shown. Light is incident on the acousto-optic medium surface from the left and exits as diffracted beams (k=0, ±1, ±2) shown in more detail in FIG. 1(b) due to the acoustic wave caused to propagate in the AO material 110 by bias applied across transducer 105. The k=0 beam represents the undeflected beam. The acoustic wave generated propagates in a direction substantially perpendicular to a direction of the light beam. In an acousto-optic Q-switch, it is usual, but not essential, for the acoustic wave to be absorbed by an absorber placed at the surface CD of the AO material 110 after passing through the region in which the optical beam is present. In the case when absorption of the wave passing through the region in which the optical beam is present is desired, it is also usual to cut the absorbing surface CD at an angle as shown to produce a wedged face in order to frustrate direct reflection of the acoustic energy back to the transducer T.
In the case considered herein, the optical beam processed by modulator 100 is a natural mode or combination of natural modes of a laser cavity and as such has a reasonably well-defined k-vector. If such an optical beam is incident on the optical modulator at or near to the so-called Bragg angle, then a proportion of that light will be converted into one or more diffracted orders as a result of the acousto-optic interaction. The exact performance of such a device in terms of its loss modulation, speed of switching, RF drive power requirements, and angular acceptance can be predicted using standard acousto-optic theory. Of particular importance is the so-called Debye-Sears ratio (often referred to as the Q-parameter), which is a dimensionless number depending on the physical length of the acoustic transducer (L), the optical wavelength (λ), the acoustic velocity (V), the refractive index (n) and the RF drive frequency used (f) according to:
                    Q        =                              πλ            ⁢                                                  ⁢                          Lf                                                                                ⁢                2                                                          nV                                                                      ⁢              2                                                          (        1        )            
Low values of Q indicate that the modulator is working in the Raman-Nath regime where multiple diffracted orders are produced, and the angular acceptance of the modulator is large. This is desirable for example if the laser is multimode leading to optical beams having a larger divergence angle. Larger values of Q (e.g. in excess of 10) mean that the device is working in the Bragg regime where a single diffracted order is produced. This is not a problem in a Q-switch, because all that is desired is to achieve loss modulation, however the angular acceptance of the modulator will be reduced in the Bragg regime, making alignment more critical and possibly truncating the angular extent of the input optical mode(s).
Another important parameter in acousto-optic devices is the so-called M2 figure of merit defined as:
                              M          2                =                                            n              6                        ⁢                          p              eff              2                                            ρ            ⁢                                                  ⁢                          V              3                                                          (        2        )            where ρ is the density of the material used and peff is the effective strain-optic coefficient. The strain-optic coefficient is constructed from the individual tensor components and depends on the orientation of the crystal, the direction and type of the acoustic wave, and the state of optical polarization. The M2 parameter has the dimensions of inverse intensity, i.e. m2W1, and the expression for the diffraction efficiency of an acousto-optic modulator produced by a given RF power level always contains the dimensionless term M2I, where I=ηPRF/area is the acoustic intensity. PRF is the drive power, η is the transducer efficiency and “area” represents the area of the acoustic transducer, which for a rectangular transducer of length L and height H will be L×H. Thus large values of M2 are desirable, as a doubling of M2 value leads to a halving of the RF power requirement for a given diffraction efficiency and wavelength.
In general, the amount of RF power required to maintain a given diffraction efficiency (and therefore a given level of loss modulation) grows with the square of the optical wavelength:PRF∝λ2   (3)as explained, for example, in “Acousto-Optic Devices: Principles, Design and Applications”, J Xu and Stroud, Wiley Series in Pure and Applied Optics, ISBN: 0471616389, 1992, high M2 materials are particularly important for acousto-optic devices operating at longer wavelengths.
As an example, fused silica (SiO2) is often used as the acousto-optic interaction medium for Q-switch lasers operating around the 1 micron region. Such lasers typically require several tens of Watts of RF drive power to achieve sufficient loss modulation to hold the laser in the “OFF” state. Typical transducer lengths are 45 mm and heights are 10 mm. The M2 of fused silica is approximately 1.51×10−15 m2W−1. Even if silica remained optically transparent at 3 microns, nearly an order of magnitude more RF drive power would be needed to provide similar performance.
Thus what is needed is an acousto-optic modulator that supports shorter (i.e. a smaller value of L) AO crystal in order to keep the Q-parameter low and to be able to fit into shorter laser cavities often used in mid infrared lasers. These shorter modulator devices must still have adequate loss modulation to hold the laser in the “OFF” state. Generally this is incompatible with keeping RF drive powers low, particularly in view of the relationship given in equation (3).